Hand-operated pumps of this type are known and widely used in hydraulic steering control units, particularly for marine engines, especially outboard engines.
The rotation of the drive shaft in either direction of rotation, e.g. by means of a steering wheel or a helm mounted thereto, generates a displacement of the pressurized fluid in a closed-loop hydraulic circuit in either direction of rotation. The pressurized fluid is supplied from the check valve to each of the two inlets of a double-acting hydraulic cylinder respectively, the latter moving along a shaft which is held stationary between two fixed points connected at its ends, in one direction or in the opposite one, according to the direction of rotation of the drive shaft of the pump. The cylinder is in turn connected by a leverage system to a steering control element, for example of a marine outboard engine or to a rudder control lever.
These prior art devices require high operational accuracy to ensure continuous, smooth operation, with no idle strokes and repeatable steering settings. This reflects on pump construction, which must provide the required pressure differences on both conduits for connection to the double-acting hydraulic cylinder, in spite of a very low rpm, because the shaft is manually operated and the rotation speed and the stroke of the steering wheel has to ensure maximized comfort and reliability.
Therefore, those pumps are relatively complex and expensive, as they are composed of relatively complex parts, having complex conduits for the passage of the pressurized fluid.
In prior art pumps, the valve plate is generally made of one piece with a closing base of the housing case, which consists of an overturned cup-shaped element having a central through hole for the end of the drive shaft to project therethrough and to be rotatably coupled to the steering wheel or the helm. The check valve is in turn sealingly secured to that base. The manufacture of the valve plate, with the holes and ports for the passage of pressurized fluids, in a single construction piece, i.e. in one piece with the base, is generally relatively expensive both in terms of fabrication of the blank and for the required drilling of the passages for the pressurized fluid, as well as for additional finishing steps.
A valve plate integrated with the base has a very high cost. Also, it requires use of a material and a surface treatment that will both ensure hardness and durability and equally conform to the stainless requirements of the base.
The valve plate generally has two eccentric axial holes, which open on the side of the valve plate associated to the rotor, into diametrically opposed radial recesses of the valve plate, which extend to almost half of the section of the valve plate and are separated by a thin intermediate diametric partition.
Those holes and these recesses are also not easily formed, when the base is made of one piece with the valve plate. Furthermore, the base must be also formed with various shapes and apertures for the passage of both the pressurized fluid and the screws that fasten it to the cup-shaped part, as well as for additional seats or receptacles, e.g. for the bearings that support the corresponding end of the rotor.
As mentioned above, prior art piston pumps must be particularly efficient and provide immediate responses even to the slightest rotation angles of the control member.
Due to these requirements, a high degree of accuracy is required for construction and assembly of the above mentioned parts, which will strongly impinge on manufacturing and maintenance costs for the pumps.
Therefore, there exists a yet unfulfilled need in the art for a piston pump that can resolve the above described drawbacks of prior art pumps, particularly a piston pump that can provide the same efficiency as prior art pumps while affording low manufacturing costs and easy construction.